ABSTRACT
This study focused on the effect of pressure sensation from the each plantar surface of the feet on postural control. The plantar surfaces of the feet were made less sensitive by cooling, using a specially designed apparatus set on a force plate. Three areas were cooled: the plantar surface of the heel, the forefoot, and the entire plantar surface of the foot. And the non-cooling condition was the control. The subjects, seven healthy men, were asked to track a continuously moving target spot displayed on a visual monitor while standing on the force plate. This tracking was done by controlling the center of foot pressure (CFP) by leaning forward and backward at the ankles. The target was moving at 0.025 Hertz (once per 40 seconds) with a triangular waveform. The moving range of the target was from 30 to 70 percent (%) of the total foot length from the heel, and this range was divided into 10 percent (%) subranges. Postural controllability was evaluated by the difference between movements of the CFP and target for each subrange. When the entire surface of the foot was cooled, postural controllability of moving the CFP anteriorly was significantly worse than the control. Postural controllability of moving the CFP anteriorly for the anterior and the posterior moving subranges was significantly worse than the control when the heel was cooled. When the forefoot was cooled, postural controllability of moving the CFP anteriorly for the anteriorly moving subrange was significantly worse than that of the control. These results suggest that pressure sensation from the plantar surface definitely participates in moving the CFP anteriorly for postural control. When the CFP is situated on the heel, pressure sensation from the heel alone may play a necessary role for postural control. When the CFP is situated on the forefoot, however pressure sensation from the forefoot may need to be the supplemented by sensation from the heel for adequate postural control.
ABSTRACT
The purpose of this study was to evaluate the relationship between the position of the center of foot pressure (CFP) and control of standing posture in ten healthy men by tracking the CFP to the moving target.<BR>Subjects were required to track a continuously moving target displayed on a screen while standing on a force plate. The velocity of the target movement was 0.05, 0.10, and 0.15 hertz (Hz) with a triangular waveform. The target was moved 30-70% within the range of the heel regarding the foot length as 100%, and the range was divided by 10%. Each subrange was named (A), (B), (C), (D) for backward movement from 70% to 30%, and (E), (F), (G), (H) for forward movement from 30% to 70%.<BR>The standing posture control was analyzed by mean error and absolute error at turning point between target- and CFP-movement.<BR>The mean error of backward movement was significantly greater than that of forward movement in the two forward subranges at 0.15 Hz. As for the other frequencies, there was no significant difference between directions of CFP movement. Mean error of A was significantly greater than that of B and C in higher frequencies, but there was no significant difference at 0.05 Hz. Regarding absolute error, there was no significant difference between the forward and backward turning point with a decrease in frequency. Mean error of D or E was greatest among the section in all frequencies. The ratio of the greater D or E and A or H was greatest at 0.05 Hz.<BR>These results suggested that CFP tracking at 0.05 Hz is hardly influenced by direction and velocity of the target movement. Terefore, we conclude that controllability of the stand-ing posture differs markedly in various CFP positions. In addition, control of the standing posture in a backward direction is inferior to that in a forward one.
ABSTRACT
A study was conducted to investigate the effects of a sagittal position of the body gravity center (GCP) and manual weight-load on postural control during rapid arm-lifting. The subjects were five males aged 21 to 36 years. They stood on a force plate while maintaining the GCP at 30%, 45% and 60% from the heel, regarding the fool length as 100%, and anteriorly lifted both arms spontaneously as rapidly as possible. These trials were carried out ten times under a 5 kg weight or no weight. EMGs of the biceps femoris muscle (BFM) and anterior deltoid muscle (ADM), the fluctuation of the center of foot pressure (CFP) and body motion were analyzed by focusing on their time sequences.<BR>At 45% and 60% GCP the BFM action started prior to the ADM action, whereas at 30% GCP it tended to lag behind. The lag times under no weight were 13.9±12.75 ms (mean±SD) at 30% GCP, -32.7±18.18 ms at 45% GCP and -46.0±19.40 ms at 60% GCP. Those under 5 kg weight were 15.0±11.40 ms at 30% GCP, -22.0±6.74 ms at 45% GCP and -28.9±7.63 ms at 60% GCP. These results indicate that the anticipatory action of the muscle related to postural control arises only at specific GCPs.<BR>The difference of starting points for BFM action to ADM action showed no significant difference between 45% and 60% GCP for either as 5 kg or no weight. The CFP position moving in a forward direction during arm-lifting showed a marked difference between 45% and 60% GCP. The time for arm-lifting showed a marked difference between 5 kg and no weight. These results suggest that the starting point of anticipatory muscle action related to postural control does not change according to the magnitude and time course of the distance to the body equilibrium as a result of arm-lifting.